Molybdenum sulfide catalysts supported on an HY zeolite at various Mo contents were studied. Catalysts were prepared by incipient wetness impregnation with ammonium heptamolybdate solution and calcined without drying. Their reactivity has been evaluated in toluene hydrogenation under typical hydrotreating conditions. Compared to alumina supported catalysts, zeolite supported Mo catalysts are extremely active for aromatics hydrogenation. At low molybdenum loading, molybdenum sulfide phases inside the zeolite show a particularly high intrinsic activity. This activity can be attributed to molybdenum sulfide clusters differing from MoS2 slabs.